The present invention relates generally to acoustic liners for suppressing noise in aircraft engines and more particularly to an erosion-resistant coating for such acoustic liners.
Aircraft engine noise can be a problem in high population areas or other noise controlled environments. In the case of turbofan engines, current design practice focuses on reducing noise through the use of absorptive acoustic liners that line various ducts of the engine, particularly the fan duct. Absorptive liners are known in various configurations, including the use of a honeycomb core sandwiched between an imperforate backing sheet and a perforate facesheet having a small amount of open surface area. This particular combination is sometimes referred to as a single degree of freedom absorptive acoustic liner.
Such liners are successful because they absorb the acoustic sound waves that impinge thereon and reduce the level of sound waves radiating from the duct terminations. Specifically, pressure waves cause air to pass into and out of the openings of the perforate facesheet, thereby generating a sufficient amount of friction, which is dissipated as heat energy. It should be understood that the key design parameter for this type of absorptive acoustic liner is the acoustic impedance, or the ratio of acoustic dynamic pressure to acoustic velocity, obtained at the surface of the liner. The acoustic impedance at any given frequency is obtained as a result of the particular configuration of the resistive and reactive elements incorporated into the mechanical design of the acoustic liner. Resistance relates to the liner's ability to dissipate noise energy as heat. Reactance relates to the liner's tendency to react noise energy back onto itself.
A modification to such single degree of freedom acoustic liners was subsequently developed to improve acoustic performance at all engine operating conditions. This modification involved the application of a woven fabric or mesh over the outer surfaces of the perforated facesheet. Typically, the acoustic mesh component is woven from fine stainless steel wires. Such liners having a mesh applied thereto are commonly known as linear single degree of freedom acoustic liners.
It is common in single degree of freedom absorptive acoustic liners to form the facesheets of lightweight composite materials to minimize the overall weight of the liner. However, many engine locations where the liners are used can present erosive environments. This is particularly true in the aft fan duct where the liner can be exposed to high-speed ingested sand particles and other debris. Composite components are highly susceptible to erosion in such environments. Because of this, the acoustic meshes that were added to improve acoustic performance were found to serve a secondary function of providing sufficient protection to perforated composite sheets. When further advances in single degree of freedom designs obviated the need for acoustic meshes to improve performance, the meshes were retained in composite liners in erosive environments because of their protective capability.
The mesh is ordinarily secured to the facesheet with a light film of epoxy adhesive. To insure that the adhesive film is uniformly thin, the use of elaborate robotic-controlled spray equipment is often used. Otherwise, excessive adhesive will wick into the mesh and result in blockages that adversely affect the performance of the liner. There are also epoxy spray misting processes that are used to acoustically tune the installed mesh. The fine epoxy droplets adhere to the mesh resulting in airflow perturbations that increase acoustic resistance of the liner.
In addition to adding complexity to the installation process, the mesh is a relatively expensive item that adds cost and weight to the liner. Furthermore, the mesh is susceptible to damage or loss during service and tends to become dirty and acoustically blocked over time. Since there is no effective cleaning technique for blocked acoustic mesh, performance recovery requires its removal and replacement in service.
Accordingly, there is a need for an absorptive acoustic liner for composite nacelle components that is relatively easy to install and tune and that eliminates the need for a protective woven screen or mesh.